Optical sensors are used in a variety of industrial applications. One such application is the production of paper. In the production of paper, several measurements are required during the manufacturing process to ensure consistent quality of the final product. Two such measurements are consistency and brightness of the paper slurry.
During the manufacture of paper, wood fibers are separated from bulk wood by either mechanical or chemical means, or a combination. Water is mixed with wood fibers to form a wood pulp slurry. In order to achieve some measure of quality control during the process, it is essential to know the ratio of wood fibers to total mass (consistency) at every step in the process.
Some sensors used in the industry to measure consistency are mechanical in nature. One early method required a calibrated tapered rod about six inches long which was dropped from a vertical position extended a distance above the pulp. A reading was then taken of the depth to which the rod sunk in the pulp. Other mechanical sensors, by one means or another, measure the force which moving slurry produces on a mechanical arm, plate, or the like. Some limitations of these mechanical sensors are distortion due to velocity of the slurry, different wood species and drainage. Also, such mechanical systems cannot be readily installed in a tower or chamber through which pulp slurry is moving slowly, or in which pulp slurry is contained. Finally, accuracy of mechanical sensors is limited for certain pulp consistencies.
The quality of paper is also dependent on the brightness of the pulp. The final color of the paper can be predicted by using a measurement of the brightness of the pulp. Traditionally, instruments generally available and used for such brightness measurements are performed on an off-line basis. In this type of instrument, a sample is periodically taken from the pulp washer, dried, and its brightness determined from a reflectance meter. This determination can take 20 to 30 minutes.
Most modern methods of determining consistency and brightness of the pulp employ some means which emit radiant energy in the direction of the pulp. The magnitude of the energy which either passes through the pulp or is reflected back from the pulp is indicative of its brightness and consistency. This reflected energy can either be measured or compared to the magnitude of the energy emitted to determine the consistency and brightness of the pulp. The magnitude of the reflective energy can be separated into components for both consistency and brightness.
In an example of an on-line system, two sensors are positioned in the pulp to measure brightness of the pulp entering and leaving a bleaching stage. The two sensors measure the intensity of back scattered light and then compare results. The differential signal is used to control bleaching chemicals in order to optimize brightness of the pulp.
An important step in determining brightness and consistency of pulp is converting the intensity of the reflected or back scattered light into a useful measurement. One drawback of prior art systems is that the intensity of the reflected or back scattered light is measured only over a limited wavelength band. This limits the accuracy the sensor by eliminating significant amounts of data. Another drawback of prior art systems are inaccurate data models. In order to generate meaningful brightness and consistency measurements, mathematical models must be generated which accurately depict the behavior of the sensor over a particular range. With mathematical models, it is important that they are first accurate, and that their accuracy span a large range of measurements.